Method and apparatus for generat



Od- 22, 1940. L w w M5 Re. 21,607

AND APPARATUS FOR GENERATING ELECTRICAL WAVES METH'J'D 4 Sheets-Sheet 1 Original Filed June 27, 1935 //v wswrom Oct. 22. 1940. A. w WILLIAMS Re.'2

u's'rncn AND APPARATUS FOR ennmnms'swcwmcm. WAVES Original Filed June 27, 1935 4 Sheets-Sheet 2 A rromvn Oct. 22. 1940. L w w AM5 Re. 21,607

' IBTHSD AND APPARATUS FOR GENERATING ELECTRICAL WAVES Original Filed June 27, 1935/ 4 Sheets-Sheet 3 nnunn T I!!! unnnnn "uni" Oct. 22'. 1940. A. L. w, WILLIAMS IBTHDD AND APPARATUS FOR GENERATING ELECTRICAL WAVES Original, iied June 27, 1935 4 Sheets-Sheet 4 nnuunlnu IIIIIIIIIIII Illlllll IIIIIIVVV lllllllllll Reissued Oct. 22, 1940 UNITED STATES METHOD AND APPARATUS FOR GENERAT- ING ELECTRICAL WAVES Alfred L. W. Williams,

Cleveland, Ohio, assignor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Original No. 2,126,437, dated August 9,1938, Se-

rial No. 28,746, June 27. 1935. Application for reissue August 19, 1939, Serial No. 291,074

23 Claims.

The invention relates to methods and apparatus for generating electrical waves corresponding to fluid medium waves, such for example as sound waves, and particularly methods and ap- 5 paratus in which there is a selective action with respect to the direction from which the fluid medium waves approach.

An object of the invention is to provide an improved method of generating electrical waves 10 corresponding to fluid medium waves in which the amplitude of the electrical waves generated varies with the direction of approach of the fluid medium waves.

, More specifically, a further object of the invention is to provide an improved method of generating electrical waves corresponding to fluid medium waves in which the amplitude of the electrical waves generated varies with the direction of approach of the fluid medium waves from go a maximum in one direction to a minimum in the opposite direction.

Another object of the invention is to provide improved apparatus suitable for use in carrying out methods such as those above referred to.

A further object of the invention is to provide a microphone having uni-directional operating characteristics.

Still another object of the invention is the provision of a microphone adapted to operate alternatively with'uni-directional, non-directional, or bi-directional characteristics.

Another object of the invention is the provision of a microphone having uni-directional operating characteristics and further characterized by a flat response throughout a relatively wide range of frequencies.

A further object of the invention is the provision of microphones having operating characteristics'such as above referred to and which are 0 further characterized by structural simplicity, compactness, light weight and relatively low cost of production.

My improved methods involve essentially utilizing the diflerent instantaneous wave pressures of the fluid medium in suitably disposed adjacent zones to generate electrical waves whose instantaneous values are proportional to the differences between the said pressures. For fluid medium waves approaching the said zones in two opposite directions the differences in instantaneous pressures are at a maximum while for fluid medium waves approaching from intermediate directions the differences in pressure between the two zones are at a minimum for directions of approach midway between the first named directions, Thus the generation of electrical waves is selective with respect to the direction of approach of the fluid medium waves with a maximum generation or response in two opposite directions. By utilizing the pressure variations of the fluid medium in a third zone, suitably disposed in relation to the two other zones, to generate corresponding electrical waves and properly combining the latter set of waves with the set of waves generated by the different instantaneous pressures in the two other zones so that the combined waves are in phase, and therefore additive, when generated by fluid medium waves approaching from one'direction and are substantially 180 out of phase, and therefore subtractive, when generated by fluid medium waves approaching from the opposite direction, there is secured an electrical response that is unidirectional with respect to the fluid medium waves. The phase angle between the two said sets of waves is substantially greater than zero and substantially less than 180. Accordingly, in order to combine the waves properly so that unidirectional response will be obtained, it is necessary-to shift the phase angle between the two sets of waves so that the waves of the two sets shall have the above mentioned in-phase relation for fluid medium waves approaching from one direction and 180 out-ofphase relation for fluid medium waves approaching from the opposite direction. This is accomplishedby passing either of the-said sets of waves through a suitable phase shifting electrical circuit. Preferably the method further involves passing the electrical waves generated by the diflerent instantaneous pressures in two of the zones through a circuit whose transmission efliciency varies inversely as the frequency for the range of frequencies utilized so that the electrical response secured is relatively flat.

My improved apparatus comprises a generator having two diaphragms suitably disposed adjacent each other so that their movements in response to different instantaneous pressures of the fluid medium waves generate electrical waves having the bi-directional characteristics above referred to. And, in order to secure the uni-directional response characteristics above referred to the apparatus includes another generator having a diaphragm exposed and sensitive to the fluid medium waves. Further the apparatus preferably includes a circuit to which the generators are connected and which has, within the frequency range for which the apparatus is designed, a transmission efllciency for waves from the two-diaphragm generator varying inversely modifying devices, withwhich the withthefrequencyand whichcircuitisadapted to'shiftthephaseanglebetweentheoutput voltagewavawhichisadaptedtocombinethe electrical waves of the two generators so that. they are substantially 180 out of phase for iiuid medium waves approaching from one direction andsubstanflailyinpha'seforiiuidmedium wavesapproaching from the opposite direction. andis alsoadapted to combine the said voltaee waves in the latter phase relations.

My improved methods and apparatus may most convenientlybedescribedandexplainedin detailinconiunctionwitheach otherandIshall now describe the preferred and alternative forms of the apparatus illustrated in the accompanying drawings and explainmy improved methods as carried out by the use of'the said apparatus. The known types of microphone generators, such as the piano-electric, dynamic, condenser and carbon types, may be employed in carrying out my invention, although I prefer to make use of the sound cell type of piezo-electric generators because of their remarkable compactness and Mel-- ityofoverawiderangeoifrequencles.

In the accompanying'drawinss, Fig. 1 is a side, elevation of a preferred form of my improved microphone in which piece-electric generators are employed.

Fig. 2 is a front elevation of the same with a foraminous supporting and enclosing casing shown in dotted. lines.

Fig.3isaplanviewoi'thesaidmicrophmier Fig. 4 is'a plan view of one of the generator lmits of the microphone with a portion of the structure brokenaway to better show the constructim.

8.5iaasectiononthelihe$-4ofl"lg.4.

Fig.0isadiagramshowingthemicrophone and a circuit. comprising suitable amplifying and microphone may be connected in me.

Figs. 7 and 8 arediagrammaticviews showing certain ofthe microphone'generatorunitsand themannerinwhichthefluidmediumwamact Figs.9andl0aretic-viewa oi modiiledformsoi'microphoneinwhichareemnloyed gmeratorunitsotthepieso-electrictypebutarrameddiil'erentlythanaretheunitsofthe ahowninl'igaLflandS.

Iig.11isaplanviewoi'amodi1ledformot pieao-electricgmerator unit, with aportion of thestructm'ebrokenawaytobettershowdetails of.

m. 12 isaaectlon on theline l2-l2 of Hg. 11. Pig.18isashowingamicrophone generator unii: of the-condenser type andacircuiticomprising amplifyingandmodifyingdevicestowhichthemicmphoneiseon- Fig.14isafiontelevationofstillanotherform orutilizing'pieso-elech'icgmerating imitsandunbodylna'myinvention. y l'ig.15isaaectiononthelinel0-llof1'ig.l4.

Fig.15aisasectionalviiewofaunit of-thepiso-electrictypewhichisaimilarto' theunitahowninmgaiandlielceptthatthe electflcalconnectiomoi'thetwocrystalelements Hg. idisadiagrammaticviewshowingthe generatorsofthemicrophoneshowninl igs. 14 andl5andacircuitcomprisingamplifyingand modifying devices, to which the microphone ism. 17and18aredi 8ram8 m 'landBbutappllcabletothe form ofmicrophone shown in Figs. 14 and 15. 15g. IDisadiagramshowinganotheriorm'o microphone having condenser type generator unitstheimprovedfeaturesofmyinvention together with a circuit comprising amplifying" and modifying devices to which themicrophone is connected.

Fig.20isaplan viewofanotherformofpiezoelectric microphone suitable for use in carrying out my invention. with a portion oi the structure broken am to better show details of construc-.- tion.

Fig.21isasectionontheline2l-2ioi1'1g. 20.

Fig.22isasectionontheline22-2I of Fig. 20.

Referring first to Figs. 1 to 6, the microphone compriss two identical generators designated in their entireties by I and In and a third generator ted in its entirety by I. The generator 2 consists of a plea-electric sound cell generatorunitsuchasisfullyshownanddescribed'in the application for United States Letters Patent of C. B. Sawyer. Serial No. 658,649 flied February 25, 1033. now Patent No. 2,105,010, issued Jan. 11, 1 88.11!!! in British Iettm's Patent No. 409,040. Bchoi'thesedisclosurespresentsfull detailsof theanditwillsumcei'orthe purposeaofthepresentinventiontostate that the sound cell generating unit which is dted in it 'mtirety by the numeral 3 comprises a framestructurelofBakeliteorthelikehaving 'two inwardly extending lugs 44 upon the opposite sides of which are supported two plate-like crystalline pieso-electric generating elements l.ldoftheflexingtype. Thecrystalelements 5, in are secured in place and tightly sealed withintheframebythhisheets iJoofpaperor the like which are cemented to the faces of the crystal elements and the rrame. The electrodes .ofonepolarityoftheeiementslIaareconnectedbyconductonland 'I.respectively,to a common conductor 0. while their electrodes of the opposite polarity are connected by conductors The flexible .Ill'lll me cated in Figs. 1, 2 and 3 by a dinits amplifying devices.

ductor I of the generator 2 and the terminal beingconnected to the grounded frame II.

The microphone generators and their supporting frame are mounted in a supporting. and protecting casing shown by dotted lines in Fig. 2 and designated in its entirety by I3. This casing preferablycomprisestop and bottom sections Ila, lab, respectively, which are Joined by a band I3c which extends horizontally around the casing. The frame II is suitably supported in the casing by blocks 20, 20 of felt, sponge rubber or the like. The casing parts Isa, lib are made of wire screen or other foraminous material so as to freely admit the air or other fluid medium.

In the use of the microphone shown in Figs. 1, 2 and 3 and above described, it preferably is connected with a circuit comprising suitable modifying devices and preferably also suitable Such a circuit is illustrated in Fig. 6. As shown in Fig. 6 the terimnal Ii of the microphone is connected to a two-stage amplifier comprising vacuum tubes 2I and 22 and their conventional associated filament-supply batteries 23, 23, grid-bias batteries 24, 24, and plate-supply battery 25, grid-blocking condenser 23, plate-coupling resistance 21 and gridleak resistances 28, 23a. In addition a condenser 23 is connected between the plate and filament of the tube 2| and serves a purpose which will later be explained. Terminal H of the microphone is connected to an amplifying tube 30 which is provided with filament and grid batteries 3| and 32, respectively, and a grid-leak resistance 33 and is also connected with platesupply battery 25 through a transformer 34 which is a common coupling device for the two ampliflers. Transformer 34, in addition to coupling together the two amplifying circuits, provides means for conducting the amplified waves to a loud speaker, phonograph recorder or any other device with which microphones are associated. Terminal I3 is common to the cathodes of the two amplifying circuits. A three-position switch 35 is provided to short circuit generator 2 or generators I and la to alter the directional characteristics of the apparatus as will later be more fully explained.

While I have shown, in Fig. 6, an amplifier circuit embodying three tubes of the triode type with directly heated cathodes and with separate filament and grid batteries, it will be understood that other known types of tubes and sources of operating potential may be employed. For instance, the tubes may be of 'the indirectly heated or heater type and, if desired, heated by alternating current. Furthermore they may be of the screen grid or pentode type and two or more tube functions may be combined within a single tube envelope. Also a common battery or other source of potential may be used to supply all of the tubes and the grid bias may be obtained from the voltage drop caused by the plate current flowing through a resistance. Sim ilarly, other expedients well known in the art may be employed in lieu of the arrangements shown in Fig. 6.

In explaining the operation of the apparatus above described, it may at the outset be assumed that the switch 33 is set in open position as shown in Fig. 6. The manner in which the fluid medium waves act upon the generators I, la and 21s illustrated in part by Figs. '7 and 8. these two figures the generator In is omitted as it is a duplicate of the generator I and functions in the same manner both when considered alone and when considered in combination with the generator 2.

In the operation of the microphone, within a relatively wide range of frequencies, the unitary generator 2 responds substantially uniformly. to fluid medium waves approaching from any direction, the range of frequencies depending chiefly upon the dimensions of the generator. The same thing is true of the individual generator units 3, 3 of the generator I (and la) but the generator I as an entirety has decidedly directional operating characteristics due to the physical arrangement of its generating units and the opposed electrical connection of the later.

This will now be explained with reference to the diagrams of Figs. '1 and 8. In these figures the generating units 3, 3 of generator I and the generator 2 are so arranged and connected that when they are all subjected to fluid medium pressure above zero, the left hand lead of the left hand unit 3 of generator I is negative with respect to its other lead and the right hand lead of the right hand unit 3 of generatorl is negative with respect to its other lead and the left hand lead of generator 2 is positive with respect to its other lead. In other words, the polarity of the left hand unit 3 of generator I is reversed with respect to the polarities of the other two units. The dotted polarity signs associated with generators I and 2 indicate this relation. In Fig. '7, graph A illustrates the pressure distribution of a fluid medium wave approaching the microphone from the direction indicated by the arrow B. The abscissae of the graph represent space distribution and the ordinates fluid medium pressure above or below zero pressure, which is shown by the axis C. By

"zero pressure" I mean the pressure of the fluid medium when .the latter is quiescent. This pressure distribution is shown at the instant when the pressure at the center of generator I is zero. Left hand unit} is in a zone of compression and right hand unit 3 is in a zone of rarification. The effect of increased pressure on .the left hand unit is indicated by the concave shape of its faces shown in full lines. The faces of right hand generating unit 3 are shown convex because it is in a zone of reduced pressure. Since the two units 3, 3 are, as above described, arranged and connected to have opposed polarities when subiect to fluid medium pressure above (or below) zero, the output of generator I is proportional they are influenced by the pressure wave A. At

the instant shown generator 2 is in a zone of zero pressure and hence its facesare shown (by solid lines) in normal position. Since the average pressure acting on generator 2 at this instant is zero no voltage is generated by it and no p0- larity signs are shown. Dotted line D is a graph of the pressure distribution one quarter cycle later or in other words after the wave has progreased in direction B for a distance of one quarter wave length. .Units 3, 3 of generator I are now acted upon by equal pressures and the faces are shown slightly concave in dotted lines. Due to the opposed electrical connection of units 3, 3 their outputs at the instant shown cancel each other and there is no difierence of potential between terminals I3 and I3. Generator 2 is now in the zone of maximum pressure and the positionsofitsiacesareshownbytheccncavedot-j ted lines. The output of generator 2 is maximum at this instant and its polarities are shown in dotted lines.

From the foregoing paragraph itcan be seen that, for the connections and direction of fluid medium waves shown in Fig. I, when terminal I3 is taken as a reference point, terminal I1 reaches a maximum negative potential one quarter cycle after terminal I 6 reaches a maximum positive potential. In other words, the oiit-g put of generator I (terminals II and II) leads the output of generator 2 (terminals I1 and II) by 270, or 1 -8 said output by 90.

By retarding the electrical wave of generator I until the fluid medium wave has progressed from position A to position D, which represents a distance of one quarter wave length. the outputs of generator I and generator 2 may be brought180 out of phase with eachother. y In Fig. 8 a similarwave E is illustrated approaching the same microphone from the opposite direction as indicated by the arrow 1''. The right hand generator unit of generator I at the instant represented is in a zone of compression andthe left hand unit isin a zoneof rariflcation, as indicated by the full line positions of the diaphragms of the generating unit. The output of generator I at-this instant is of the same magnitude as shown for a similar wave in Fig. '1 but of opposite polarity due to the reversal of the pressures with respect to the units of generator I.

- At the instant represented generator 2 is in a zone of zero pressure audits faces are shown in normal position insolidlims. At this instant no output is produced by generator 2 and so no I faces are shown as concave, by dotted lines. Its

polarityisalsoshownindottedlinesandisthe same as for similar" conditions in Fig. 'I.

From consideration-of the foregoing description and Fig. 8 itis apparent that when terminal "is takenasare'ierencepoint,theterminal Il reaches amaximum negative potential one quartercycle after-terminal Ii reaches amaximum negative potential. In other words, the output of generator I (terminals Ii'and ll) leads the output generator 2 by 90"..

Byretarding the electrical waves at generator I until the corresponding fluid medium waves .haveprogressedfrompositionlttopositionG,

which represents a distance of one quarter wave length or a phase displacement of 90, the outputs of generator I and generator 2 may be' brought in'phase with eachother, whereas in Fig.7 retarding the output of generator! by 90 brings the outputs 180 out of By electrically retarding the output of generator I and then combining the med output with the output of generator 2 the combined response to fluid medium waves may be made-to varyasthedirectionofthewavesischanged from a maximum for waves in one direction to a minimum for waves in the opposite direction.

ar,eor

For waves arriving from direction B in Fig. 7 the retarded output of generator I will tend to cancel the output of generator 2, providing a minimum response. For waves arriving from direction F in Fig. 3 the retarded output of generator I will reenforce'the output of generator 2, providing a maximum response. Reversing the polarity of generator I or generator 2 will reverse the directions of maximum and minimum response.

For fluid medium waves approaching the microphone from any direction at right angles to the directions indicated by arrows B and F of Figs. '1 and 8, units 3, 3 of generator I are affected equally and simultaneously and hence, due to their opposed connection, their outputs are cancelled and the output of the microphone is that of generator 2 alone. For fluid medium waves approaching the microphone in directions intermediate between those last referred to and the direction B the output will vary from that of the generator 2 to the minimum output for waves in the direction B. And similarly, for fluid medium waves approaching the microphone from directions intermediate between the direction at right angles to directions B and F and the direc-' tion F the output of the microphone will vary from the output of the generator 2 to the maximum output of the instrument for fluid waves approaching from the direction F.

As an alternative method of bringing the outputs into phase for waves from one direction and out of phase for waves from the opposite direction, the output of generator 2 may be retarded instead of the output of generator I. In this case the outputs will oppose for waves arriving as shown in Fig. 8 and will aid each other for waves arriving as shown in Fig. 7. The directions of maximum and minimum response will then be.

' generator I from either direction B or direction F, we flnd that the maximum instantaneous pressure diflerence will be very small for very long waves and will increase as the wave length decreases'until the wave length equals twice the center to center distance between the units 3, 3. Atthiswavelngthwhenone unit3is at a point of maximum pressure in the sound-,wave. the

otherunitlisatapointof minimumpressure and hence the pressure difference is maximum.

As the wave length is further decreased the difof generatof'l to a'corruponding point on the other unit 3 of generator -I. The apparent centertocenterspacingequalsthetruecenterto center spacing for sound waves arriving from direction 8 or direction F of Figs. 7 and 8 and approaches zero as the direction of the sound waves approaches the perpendicular to direction 3 or direction 1'.

Stated in another way, maximum response occurs for waves whose period is equal to twice the time required for sound waves to travel from a point on one unit 3 of generator I to a corresponding point on the other unit 2 of generator I. As the period of the wave is further reduced the response drops, reaching uro when the period is equal to the time required for a wave to travel from a point on one unit I to a corresponding point on the other unit 2 of generator I.

The above relation may be expressed In the form of equations. If we let P,,,=maximum instantaneous wave pressure d =apparent center to center spacing of units 3, 3 Y=wave length of the fluid medium waves f=frequency of the fluid medium waves=% where o=period of the wave P =maximum instantaneous diilerence in pressure between corresponding points on units 3, 3 at frequency f v=velocity of propagation in the fluid medium Then we have (1) P =2P,.. sin(% 180) 0 0r, since Y=-, we have The ratio, %is the time required for waves to progress from a point in one unit 3 to a corresponding point in the other unit I.

As an example, assume that sound waves of various frequencies 1 or periods a. approach generator I from direction B or direction F in Figs. '7 and 8, and assume that d equals .578 inch, v equals 13,000 inches per second, Pm equals 1,000 units of pressure. Then the above formula reduces to .578 P =2X 1000 Blll( XfX 180)=2000 sin(.008f) and Tabulating values of Pa against I and a, we have I (cycles) 0 (seconds) P;

100 01 28 200 00s 66 400 (1)26 112 8!) (D125 m 1, 6i!) OM26 444 3, s00 M1313 ass 6, 400 (111157 1, 560 11.250 2,000

In order to obtain uniform response over the operating range of frequencies it is desirable to provide compensation for the above-mentioned increase in response as the frequency increases. In the preferred form of my apparatus I provide means for simultaneously altering the response of generator I and shifting the phase of the output of generator I relative to the output of generator 2. This response-correcting and phaseshifting means comprises condenser 29 of F18. 6 and the plate resistance of vacuum tube 2I in combination with resistances 21 and 28a. Condenser 28 is of such capacity that its reactance at the lowest operating frequency is very small compared with the plate resistance of tube 2I. The resistances 21 and 28a are very large compared with thehighest operating impedance of condenser 29. Under these conditions the voltage developed across condenser 29 and hence the voltage applied to the grid of tube 22 decreases as the frequency increases providing substantially the necessary correction for the increase in response of generator I. Also the voltage developed across condenser 29 over a wide range of frequencies is substantially 90 degrees out of phase with the voltage applied to the grid of tube 2I by generator I. Hence the output of tube 22 for constant amplitude input varies inver ly as the frequency varies and is 90 degrees at of phase with the output of generator I. This decreasing response of the amplifier taken in combination with the increasing response with frequency of generators I and. la results in a substantially uniform output of tube 22 as the frequency of fluid medium waves falling on generator I is varied. The output of generator 2 is amplified by vacuum tube 30 without otherwise being altered. And since the output of generator 2 is practically uniform with frequency, the output of tube 30 will be-substantially uniform. The outputs of tubes 22 and III are combined in transformer ll.

By proper adjustment of potentiometer 28a. in Fig. 6 the amplified and corrected output of generator I may be made equal in magnitude to the amplified output of generator 2 for waves arriving from either direction B or F of Fi s. 7 and 8. Then for waves arriving from one of the above mentioned directions the outputs will cancel and for waves arriving from the opposite direction the outputs will combine to a value of twice that of either output alone.

From the foregoing description it will be seen that in cases where a non-directional pickup characteristic is required the generators I, la

may be disconnected or short circuited allowing the non-directional generator 2 to function alone, thus obviating the use of a separate microphone. Furthermore. since the generators I and la have maximum response to waves from two opposite directions as previously explained and have no responseto waves approaching from directions at 90 degrees to the directions of maximum response my microphon'e may be used for bi-directional pickup by disconnecting generator 2. These results are secured in the apparatus as illustrated in'Fig. 6 by moving the switch 35 to suitable positions, that vis, to short circuit the preferably being connected with an amplifying 7 circuit including response correcting means such as the condenser 2!.

As previously explained, the response of the generators I. la rises with the frequency up to the point where the period of the wave becomes twice the time required for a wave to travel from one unit 3 to the other unit I of generator I. At higher frequencies the response decreases reaching zero at twice the frequencyof maximum response. Thus itcanbeseenthattoobtain substantially uniform response with simple correcting means, such as condenser 29, generating units must be small enough to be placed one-half wave length apart at the highest operating frequency. Thus for a smooth response up to ten thousand cycles per second the space should be no greater than 0.65 inch. However, the theoretically optimum spacing specified can be departed from substantially and still secure reasonably satisfactory results since the response does not drop to zero until twice the optimum spacing is reached.

Since microphones in service are used with varying lengths of connecting cable, it is desirable, in, some cases at least, to have the internal impedance of the bi-directional and non-directional generators equal so that various lengths of cable affect both generators equally. Since the impedance of generators I and In each comprise two generating units in series it is necessary to connect two generators I and la in parallel to obtain the same effective internal impedance as that of generator 2. Obviously other methods of connection can be employed. For instance, the generating units 3, 3 of generator I can be connected in parallel in opposed relation and the generating units of generator Is likewise connected. Thai, to maintain the impedance balance, generators I and la would be connected in series. Obvi'ouslymore than one generator 2 can be employed in combination with larger numbers of generators I, la, etc. For many uses, as for example in acoustical work, generating unitsofquitesmallsiaearerequiredtopermit the 11 close spacing of such units in order to secure'a flat response over relatively wide ranges of frequency and I have foimd that the pieso-eiectric generator units of the sound cell type illustrated in Figs. 1, 2 and 3 admirably meet these requirements.

While the t of the generator units showninFigs. 1,2 aud3 issuchaslprefer, various other arrangements can be employed. Thus inthemodifiedmicro oneshown'inl 'ig.9the arrowsB andl in tethe directionsof the fiuid medium wam for maximum response of the bi-directional generators II and "a which have their generating imits la, In at right angles to the directions B 1' instead of parallel thereto, the generator 81 having its generating unit is parallel to the directions 3 ,1 The generators and their units are connected electrically as shown in Pig. 9 and the instrumenthastlireeterminals Ila, Ila, liq corresponding to the terminals of the instrument showninl'ig.6andtheseterminals Ita, "a and llamaybeconnectedwiththeamplifiercircuit illustrated in Fig. 8. The construction of the unltslaisidentical with that of the units 3 already described.

Simflarly, with modification shown in Fig.

10 the arrows B and I represent the directions of approach of the fluid medium waves for maximum response of the Iii-directional generators is and Ila and in-this case the'non-directional generatcrtlaswellasthegeneratingunitstmwd the generators ll and "a, is disposed at right angles to the said directions. The terminals ISb, I'Ib and I lb of the microphone of Fig. 10 also may be connected to the amplifying circuit shown in Fig. 6.

While I prefer to employ sound cell generating units such as are shown in Figs. 4 and 5 in which two oppositely disposed crystalline generating elements are mounted on the same frame, it will be understood that sound cell units each employing a single crystalline generating element can be employed. I have shown a sound cell of this character in Figs. 11 and 12. Here the unit which is designated as an entirety by 40 comprises a cupped frame structure ll, a flexing crystalline element 42 of the same character as the elements 5, la in the previously described construction, and a flexible sheet ll of paper or the like which is cemented to the element 42 and the edge part of the frame I so as to tightly enclose the crystalline element, said elementv being operatively supported upon lugs lla on the frame ll. The element 42 has its electrodes provided with leads 4 and 45 corresponding to the leads 8 and ll of the first described sound cell. a

In Fig. 13 I have shown another form of microphone embodying my invention. Here the generators are of the condenser type and the microphone comprises a bi-directional generator 46 and anon-directional generator 41. The latter generator and each of the two units 48, 48 of the generator 46 comprises a cup-shaped base or.

frame 49 formed of some suitable insulating material, a metallic back plate 50 and a diaphragm 5| which may be of metal or of some material such as mica provided with a conducting coating opposite the back plate It. The edges of the diaphragm are tightly sealed to the top peripheral part of the base or frame 48. The operation of condenser type microphones is well known to persons familiar with the art. The diaphragms II and back plates 50 form electrical condensers whose capacities vary in accordance with the variations in fluid medium pressure acting on the diaphragm. Battery 52 electrically charges the capacities of the two generator units of generator supplies an electrical charge for the capacity of generator 41 through resistance 56. Generator 41 is equally affected by waves from any direction or, in other words, is non-directional. The microphone has terminals lie, IIc and lie to which an amplifying circuit 5 1, similar to that of Fig. 6, is connected. Condensers 58 and 59 are provided to isolate batteries 52 and 55, respectively, from the amplifier without aifecting the outputs of the generators.

It will be observed that the generating units ll, ll of the generator I and the generating unit constituting generator 41 have the same kind of space relationship as the unit of the generators I and 2 of the first described microphone and the instrument of Fig. 13 operates in the same general way as that of Figs. 1, 2 and 3. That is to flexure of the generators.

instrument and by moving the switch 350 either the generator 46 may be short circuited to render the instrument non-directional or the generator 41 may be short circuited to render the instrument bi-directional.

In Figs. 14, 15 and 16 I have illustrated still another form of uni-directional microphone in which piezo-electric crystalline generators are employed. Here the microphone consists of two bi-directional generators 80, 60a and a nondlrectional generator 6|. Each of the generators 60, 60a consists essentially of a flexible crystalline element of the same character as the elements 5, a of the first described microphone, said element being mounted upon two opposite lugs 82a of a supporting frame 62 with interposed pads 63 of felt or the like. Each generating element of the generators Bil, 600. are provided with terminal leads 64, 65 as shown in Fig. 14.

The generator 8| consists of a piezo-electric sound cell, such as is shown in Figs. 4 and 5 and need not be further described. This sound cell or generating unit is mounted in notches 62b of the frame 62 with interposed cushions 66. The generator 6| is provided with terminal leads 6! and 68 and the leads of the three generators are connected as shown in Figs. 14 and 16 to termin'als lid, Nd and ltd of the instrument. These terminals are in turn connected to an amplifying circuit 69 of the same charatcer as the amplifying circuit of Fig, 6. The circuit includes a three-position switch 351). It will be observed that the generators 60, 60a of the microphone are connected in series in aiding relation, two'generators being used to obtain greater output. The generators vGil and 60a each provides bi-directional response. This will become evident when it is considered that a wave arriving from any point in the plane of generators 60 and 600 will react on both surfaces of each of said generators equally and simultaneously, resulting in no flexure. Waves arriving from other directions will react on one surface of each generator before reacting on the other surface causing a The character of generators 60 and 60a. is' such that the maximum voltage is generated at the instant of maximum fiexure. Maximum output will be obtained when the waves arrive from directions normal to the surface of the generators ill and 60a. Thus it can be seen that they have the same 'bi-directional response as the units previously described.

In Fig. 17 I have illustrated graphically at H the pressure distribution of a fluid medium wave approaching the microphone of Fig. 14 from a 'direction indicated by' the arrow J. This wave is shown at the instant of maximum pressure on generator 6|. The faces of generator ii are therefore shown in concave position in solid lines and the resulting polarities are also shown in solid lines. At the instant illustrated generators III and Gila are acted upon by equal pressures on their opposite faces and hence at this instant there is no flexure of these generators, as shown by solid lines. The dotted line at K illustrates the same wave one quarter cycle later. Generator BI is now in a zone of zero pressure and its faces are shown in normal position in dotted lines At this instant there is no output from generator SI and so no polarity signs are shown. Generators 60 and Ma at the instant illustrated are acted upon by greater pressure from the right hand side than from the left hand side and their resulting flexure is shown in dotted lines. At instant shown the pressure diflerence is maximum and hence their outputs are maximum. Their polarities at this instant are 'also shown indotted lines.

From the last preceding paragraph it can be seen that when. terminal ltd is taken as a refer-. ence point terminal ltd reaches a maximum positive potential one-quarter cycle after terminal lld reaches a maximum negative potential. In other words, the output of generator 6| leads the output of generators 60 and 60a by 270 or lags by 90.

Fig. 18 presents a graph L of the pressure distribution of a fluid medium wave approaching the microphone of Fig. 14 from the direction indicated by arrow M. It represents the instant of maximurr pressure on generator 6|. The output of generator BI is a maximum at this instant and of the same polarity as for similar conditions shown in Fig. 17. Generators ill and 60a are acted upon equally on both faces and hence they do not flex at this instant. Dotted line N represents the pressure distribution one quarter cycle later. Generator ii is now in a zero pressure zone and its faces are shown in normal position in dotted lines. Generators 60 and 80a are now acted upon by greater pressure on the left hand side than on the right and this difference in pressure is maximum at the instant shown and their resulting flexure is shown in dotted lines. Their polarities are also shown in dotted lines. The output of generators 60 and 80a is therefore maximum but opposite in polarity to the output for conditions shown in Fig. 17.

From the foregoing description, it is evident that when terminal lBd is taken as a reference point, terminal lid reaches its maximum negative potential one-quarter cycle after terminal I'ld reaches its maximum negative potential. other words, the output of generator 6| leads the output of generators 60 and 60a by 90, whereas under the conditions shown in Fig. 17 the output of generator 6| lags the output of generators l0 and 60a by 90.

Retarding the electrical output of generators 60 and 60a by 90 degrees will bring their outputs in phase with the output of generator SI for sound waves arriving from one direction and 180 degrees out of phase for sound waves arriving from the opposite direction.

Since both faces of each generator ill and 60a are exposed to the fluid medium waves the deflection of these units at any instant is proportional to the instantaneous difl'erence in pressure on opposite faces. For plane waves of constant amplitude the pressure differential between two points, as has previously been explained in connection with the tabulated values for Equation (2) for practical purposes increases substantially linearly with increase in frequency for frequencies less than some critical value depending upon the distance between the two points. Since the outputs of generators 60 and 80a are proportional to their flexures and their flexures are proportional to the pressure diflerential which in turn is substantially proportional to the frequency, their outputs are substantially proportional to the frequency. This is the same type of response obtained .from generators and In as explained in connection with Figs. 7 and 8.

Since the response of generators 80 and 60a depends on a diflerence in pressur'eon opposite faces the response will be zero for waves approaching B0 and 60a from any direction in the central plane of the units because from such directions waves act on both sides equallyand simultaneously. The maflmum response occurs for waves approaching from directions normal to tor ii provides uni-directional pickupinthesame' manner as do the bi-directional generators I and la and non-directional generator i of ms. 1, 2 and 3.

As a modification of the construction shown in Figs. 14 and 15 the bi-directional generators ill, filo may be replaced by generators of the sound cell type of construction such as shown in Fig. 15a. The constructional this generator is like that of .the sound cell shown in lflss. 4 and 5 except that the two generating elements 8 and n in are electrically connected in opposition as will be seen from a comparison of Fig. 150 with Fig. 5. Fluid medium waves arriving from directions in the plane of the generator flex the two pieceelectric elements eq ally and simultaneously but in opposite directions, and due to the manner in which the elements are connected their output waves cancel. Fluid medium waves arriving from other directions have diflerent instantaneous efl'ects on the two elements. The outputs therefore do not cancel and the resulting output is similar to the output of generators I and Ila of Fig. 14. Thus the operation of the microphone employing such reversed sound cells in lieu of the generators il, "a isthe same as the operation of the microphone in Figs. 14 and 15.

In Fig. 19 I have shown a microphone operating in the same general way as the microphone shown in Figs. 14 and 15 but in which the generating units are of the condenser type. The microphone comprises a bi-directional generator 10' and a non-directional generator II. The bidirectional generator comprises a frame I! of suitable insulating material, a metallic back plate 13 mounted in the frame and formed with a number of perforations Ila to allow free passage of the fluid medium, and a diaphragm ll of metal or of some such material as mica with a conducting coating opposite the back plate It.

The generator ll comprises a frame ll of insulating material, a metallic back plate I! and a diaphragm H, the construction of this generator being the same as that of the generating units of Fig. 13. v

- Diaphragm l4 and back plate II of generator ll form a capacity which varies under the influence of fluid medium waves. Diaphragm II and back plate It of generator II also form a capacity whose value varies under the influence oi fluid medium waves. Battery I8 applies electrical charges to the capacities of generators Ill and H- through resistances Is and II. Since the back plate of generator II is perforated the generator will have bi-directional characteristics for the reasons explained in connection with generators Cl and a of H881! and 18 as applied to the microphone of Figs. 14 and 15. Variations in the capacities of generators II and II under the influence oi fluid medium waves cause alterhating potentials to be applied to the terminals lie, He and 18s.. Condensers II and 82 are provided to isolate battery II from terminal lie and I'Ie. These terminals are connected to amplifier circuit 83 which is of the same character as the amplifier circuit of Fig. 6. The cir- ILOOT meansoiwhicheitherthegeneratorlio'rthe generator ll canbc short circuited so that the microphone will function non-directionally or iii-directionally aswell as uni-directionalw.

Inl'igs. 20,21and221haysillustrateda bidirectional generator. in which two separate mechanical diaphrasms are-employed to. actuate a single generator imit of the crystalline piezoelectric type. In this generator there is a flexing crystalline piezo generator element 84 of the twisting type disclosed in United States patent to Sawyer Reissue No. 20,680 (Fig. 10) to which reference may be made for a detailed description. In this type of generating element the response is due to a twisting of the element and is proportional to the amount of the twisti This generator is provided with terminal leads a and Nb and is mounted in a casing comprising a cupped bottom section I and a cover plate 86 secured to the bottom section by means of cap screws 81, the parts I and It being formed of suitable insulating material. 'Ihe generating element is supported from the bottom of the case bymeansofacentrallydisposedblock It and two corner blocks .0, ll, these blocks being preferably formed of some material such as rubber,theblocksbeinlcementedtothecaseandto the crystal element. The free corners of the plate-like crystal element carry blocks 9', I to which are attached circular dia 9|. II, the peripheries of which rest pon and are cemented to the edges of circular apertures "a in the top Plate ll when the diaphragms'il, II are acted upon by diflerent pressures a twisting flexure of the crystalline element ll results. when diaphragms II, II are acted upon simultaneously by the same prmsure no twisting flexure results. Accordingly fluid medium waves arriving from directions parallel to a line Joining the centers of the diap sm act upon the two dianhr s s with different forces as shown in connection with the first described generator of Figs. 7 and 8. difierence in force results in a twisting motion of the crystalline element causing the generation of corresponding electrical output. Fluid medium waves reaching the generator from directions 90 degrees from the above mentioned directions act on the two diaphragms equally and simultaneously and produce minimum output. Thus it will be seen that this generator'has the same bi-directional characteristics as those previously described and may be combined with non-directional generators for uni-directional reception where desired.

As will readily be imderstood by those skilled in such matters, other forms of amplifying circult than those illustrated can be employed in connection with my improved microphone. In the circuits illustrated and described'I have employed a condenser, as the condenser II in Fig. 6, which cooperata with the plate resistance of an amplifying tube to shift the phase and alter the response of the bi-directional generator or generators after the output of that generator has been partially amplified. It will be obvious that this phase shifting and modifying function can be performed before any amplification of the output of the bi-directional generators by the provision 0! suitable known means for that purpose and, furthermore, the output of the nondirectional generator may becomblned with-the altered output of the bi-directional generator before-amplification.

This

cuit'includes a three-position switch Ole by My improved microphones, operating in the 15 directional generator thereof, involves the selection of adjacent zones which are deflned by the diaphragms of the bi-directional generator. When fluid medium waves act upon the diaphragms the fluid medium pressures thereon are utilized to generate electrical waves in the manner previously described.

I prefer to design the bi-directional generators so that the zones deflned by their diaphragms are disposed in relation to each other-so that the maximum time required for a fluid medium wave to travel from a common point of one of said zones to a corresponding pointof the other zone is less than the period of the shortest wave within the range of frequencies for which the electrical waves are to be generated. By means of the bi-directional generator the diflerent instantaneous wave pressures of the fluid medium in the two zones is utilized to generate electrical waves whose instantaneous values are proportional to the differences betweenthe said pressures, and the waves so generated are then passed through a circuit whose transmission efflciency varies inversely as the frequency within the said range of frequencies-so that within the said range a generating efliciency relatively uniform with respect to frequency is attained for fluid medium waves approaching from any given direction.

It will further be seen that the arrangement of the non-directional generating unit of my improved microphone involves the selction of a third zone of the fluid medium and when the instantaneous wave pressures in this third zone corresponding to the differing pressures of the two zones of the bi-directicnal' generator are utilized to generate electrical waves and the waves generated by the two generators are combined in phase for fluid medium waves approaching the zones from one direction, a generation of electrical waves uniformly eflicient with respect to frequency is attained and the response varies from a maximum for fluid medium waves approaching from one direction to a minimum'for fluid medium waves approaching from the opposite direction.

I have shown and described the preferred procedure in practicing my improved method and preferred forms of construction of my improved microphone and associated devices but it'will be understood that other speciflc procedures and other forms of construction and arrangement of the apparatus and its parts can be employed in carrying out my invention as defined in the appended claims.

What I claim is:

1. The method of generating electrical waves corresponding to fluid medium waves within a given range of frequencies which comprises selecting two zones of the fluid medium disposed in relation to each other so that the maximum time required for one of the fluid medium waves to travel from a given point of one of said zones to a corresponding point of the other zone is less than the period of the shortest wave within the said range of. frequencies, utilizing the different instantaneous wave pressures of the fluid medium in the said zones to generate electrical waves whose corresponding instantaneous voltage values are proportional to the differences between the said pressures, utilizing the corresponding 113- stantaneous wave pressures in a third zone-of the fluid medium to generate electrical waves whose instantaneous voltage values are proportional to.

the instantaneous wave pressures in said third zone, and combining the two sets of electrical waves so generated so that they are substantially in phase for fluid medium waves within the said range approaching the said zones from one direction.

2. The method of generating electrical waves corresponding to fluid medium waves within a given range of frequencies which comprises selecting two zones of 'thefluid medium disposed in relation to each other so that the maximum time required for one of the fluid medium waves to travel from a given point of one of said "zones to a corresponding point of the other zone is less than the period of the shortest wave within the said range of frequencies, utilizing the different instantaneous wave pressures of the fluid medium in the said zones to generate electrical waves whose instantaneous voltage values are proportional to the differences between the said pressures, passing the last named waves so generated through a circuitwhose voltage transmission efliciency varies substantially inversely as the frequency within the said range of frequencies, utilizing the corresponding instantaneous wave pressures in a third zone of the fluid medium to generate electrical waves whose instantaneous voltage values are proportional to the instantaneous wave-pressures in said third zone, and combining the last namedwaves and the waves passed through the said circuit so that the waves of the two sets of waves are substantially in phase for fluid'medium waves within the said frequency range approaching the said zones from one direction. I

3. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of a generator comprising diaphragm means having a pair of surfacessensitive to the fluid medium waves and so disposed in relation to each other that the maximumtime required for one of said waves to travel from a given point of one of said surfaces to a corresponding "point of the other surface is lessthan the period of the. shortest wave within the said range of frequencies, said -generator being adapted to generate electricalwaves whose instantaneous voltage values are proportional to the differences between the CO1".- responding instantaneous fluid medium pressures on said diaphragm surfaces, a second generator associated with the flrst named generator and adapted to generate electrical waves correspondphase angle between said output voltage waves to substantially 180 and combine the said waves in the latter phase relation.

4. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of waves corresponding to the fluid medium waves approaching it from any direction, and a circuit to which the two generators are connected comprising means adapted, within the said range of frequencies, to render the response of the first generator substantially uniform and to combine the outputs of the generators substantially 180 out of phase'with each other when the generators are generating electrical waves correspond- I ing to fluid medium waves approaching the apparatus from a given direction.

5. In apparatus for generating electrical waves corresponding tofluid medium waves within a given range of frequencies, the combination of a generator having a pair of diaphragms sensitive to the fluid medium waves so disposed in relation to each other that the maximum time required for a fluid medium wave to travel from a given pointof one of said diaphragms to a corresponding point of the other diaphragm is less than the period of the shortest wave within said range of frequencies, said generator being adapted to generate electrical waves whose instantaneous voltage values, are proportional to the differences between the corresponding instantaneous fluid medium pressures on said diaphragms, a second enerator associated with the first named generator and adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction,-and a circuit to which the two generators are connected comprising means adapted, within the said range of frequencies, to combine the outputs of the generators substantially 180 out of phase with each other when the generators are generating electrical waves corresponding to fluid medium waves approaching the apparatus from a given direction.

6. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of a generator comprisingm pair of generating units each having a diaphragm sensitive to the fluid medium waves, the diaphragms of the two units being so disposed in relation to each other that the maximum time required for one of said waves to travel from a given point of one of said diaphragms to a corresponding point of the other diaphragm is less than-the period of the shortest wave within the said range of frequencies, the said generating units being connected in opposed electrical relation and each being adapted to generate electrical waves corresponding to the fluid medium waves, a second generator associated'with the first named generator and adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction, and a circuit to which the two generators are connected comprising means adapted, within said range of frequencies, to combine the outputs of the generators substantially 180' out of phase with each other when the generators are generating electrical waves corresponding to flu d i iit i- I um waves approaching the apparatus from a given direction.

7. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of a generator comprising diaphragm means having a pair of surfaces sensitive to the fluid medium waves and so disposed in relation to each other that the maximum time required for one of said waves to travel from a given point of one of said surfaces to a corresponding point of the other surface is less than the period of the shortest wave within the said range of frequencies, said generator being adapted to generate electrical waves whose instantaneous voltage values are proportional to tie differences between the corresponding instantaneous fluid medium pressures on said diaphragm surfaces, a second generator adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction and so disposed in relation to the first generator that the fluid pressure upon it is zero when the difference in the pressures on the diaphragm surfaces of the first generator is a 1 maximum, and a circuit to which the two gen erators are connected comprising means adapted,

\ within the said range of frequencies, to shift the phase relation between the electrical waves of the a two generators substantially 90 degrees and to wave within the said range of frequencies, said generator being adapted to generate electrical waves whose instantaneous voltage values are proportional to the differences between the corresponding instantaneous fiuid medium pressures on said diaphragm surfaces, a second generator associated with the first named generator and adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction, a circuit to which the two generators are connected comprising means adapted, within the said range of frequencies, to combine the outputs of the generators substantially 180 out of phase with each other when the generators are generating electrical waves vcorresponding to fluid medium waves approaching X the apparatus from a given direction, and means for electrically disconnecting one of the two generators from the circuit without disconnecting the other.

9. In apparatus for generating electrical waves corresponding to fiuid medium waves within a given range of frequencies, the combination of a pair of generators each comprising two generating units each of which has a diaphragm sensitive to the fluid medium waves, the diaphragms of the two units being so disposed in relation to each other that the maximum time requiredfor one of said waves to travel from a given point of one of said diaphragms to a corresponding point of the other diaphragm is less than the period of the shortest wave within the saidrange of frequencies, the said units of each generator being connected in opposed electrical relation andeach being adapted to generate electrical wavescorresponding to the fluid medium waves, and a third generator having .a diaphragm sensitive to the fluid medium waves and adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction, the three generators being disposed in spaced relation one above the other with their diaphragms substantially parallel and with the diaphragms of one of the two-unit generators substantially in vertical alignment with the diaphragms of the other two-unit generator and with the diaphragm of the third generator symmetrically disposed in relation to a plane midway between, and normal to the planes of, the diaphragms of the two-unit generators.

10. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of a pair of generators each comprising two generating units each of which has a diaphragm sensitive to the fluid medium waves, the diaphragms of the two units being so disposed in relation to each other that the maximum time required for one of said waves to travel from a given point of one of said diaphragms to a corresponding point of the other diaphragm is less than the period of the shortest wave within the said range of frequencies, the said units of each generator being connected in opposed electrical relation and each being adapted to generate electrical waves corresponding to the fluid medium waves, a third generator having a diaphragm sensitive to the fluid medium waves and adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction, the three generators being disposed in spaced relation one above the other with their diaphragms substantially parallel and with the diaphragms 'of one of the two-unit generators, substantially in vertical alignment with the diaphragms of the other two-unit generator and with the diaphragm of thethird generator symmetrically disposed in relation to a plane midway between, and normal to the planes'of, the diaphragms of the two-unit generators, and a circuit to which thethree generators are connected com-' prising means adapted, within the said ramge of frequencies, to shift the phase relation between the electrical waves of the third generator and those of the two two-unit generators substantially degrees and to combine the said waves.

11. In apparatus for generating electrical waves correspondingqto fluid medium waves with-' v in a given range offrequencies, the combination 1 of a generator comprising diaphragm means having .a pair of surfaces sensitive to the fluid, medium waves and so disposed in relation to each other that the maximum time required for one of said waves to travel from a given point of one of said surfaces to acorresponding point of the other surface is less than the period of the shortest wave within the said range of frequencies, said generator being adapted to generate electrical waves whose instantaneous voltage values are substantially proportional to the differences' between the corresponding instantaneous fluid medium pressures on said diaphragm surfaces, a second generator adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction and so disposed in relation to the first generator that the fluid pressure upon it is zerov when the difference in the pressures on the .diaphragm surfaces of the flrst generator is a maximum,

.and a circuit to. which the two generators are connected comprising means adapted, within the said range of frequencies, to render the response of the first generator substantially uniform and to shift the phase relation between the electrical waves of the two generators substantially 90 and to combine the said waves- 12. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of a generator comprisingqa pair of generating units .each having a diaphragm sensitive to the fluid medium waves, the diaphragms oi the two generate electrical waves corresponding to thefluid medium waves approaching it from any direction and so disposed in relation to the flrst generator that .the fluid pressure upon it is zero when the difference in'pressures on the generat-, v ing-units of the flrst generator is a maximum, and a circuit to which ,the two generators are connected comprising means adapted, within the said range of frequencies, to shift the phase relation between the electrical waves of the two generators substantially 90 and to combine the said waves.

13. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of a generator comprising a pair of generating units each having a diaphragm sensitive to the fluid medium waves, the diaphragms of" the two units being so disposed in relation to each other that the maximum time required for one of said waves to travel from a given point of one of said diaphragms to a corresponding point of the other diaphragm is less than the period the difference in the pressures on the diaphragm surfaces of the first generator is a maximum, and a circuit to which the two generators are connected comprising means adapted, within the said range of frequencies, torender the response of the flrst generator substantially uniform and to shift the phase relation between the electrical l waves of the two generators substantially 90 and to combine the said waves.

14. In vapparatus for generating electrical waves corresponding to fluid medium waves within agiven range of frequencies with maximum generation for fluid medium waves approaching from a given direction, the combinationof a generator comprising a pair of generating units each waves, the two units being so disposed in relation to each other that the center to center distance between diaphragms of the two units' is'les's than the length of the shortest waves within said range of frequencies and the said generating units being connected in opposed electrical relation and each being adapted to generate electrical waves corresponding to the fluid medium waves; a second generator adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction and so disposed in relation to the first generator that the fluid pressure upon it is zero when the difference in the pressures on the diaphragms of the first generator'is a maximum; andia circuit to which the two generators are connected comprising means adapted, within the said range of frequencies, to shift the phase relation between the electrical waves of the two generators substantially 90 and to render the response of the first generator substantially uniform over the said range of frequencies and to combine the said waves.

15. In a directional microphone for operation over a given range of frequencies, the combination of a bi-directional microphone having response substantially proportional to frequency within the given range of frequencies, a second microphone sensitive to fluid medium waves approaching it from any direction and whose response is substantially uniform with respect to frequency within the given range of frequencies. the two microphones being so disposed in relation to each other that their electrical outputs are substantially 90 out of phase over the given range of frequencies, electrical circuit means connected to the bi-directional microphone and adapted to render the output of the bi-directional microphone substantially uniform and to shift the phase of said output by substantially 90, and means for combining the output of the second microphone and the altered output of the bi-directional microphone.

16. In a directional microphone for operation over a given range of frequencies, the combination of -a bi-directional microphone having response substantially proportional to frequency within the given range of frequencies, a second microphone sensitive to fluid medium waves approaching it from any direction and whose response is substantially uniform with respect to frequency within the given range of frequencies, the two microphonesbeing so disposed in relation to each 'other that their electrical outputs are substanbination of a generator comprising diaphragm' means having a pair of surfaces sensitive to the fluid medium waves, said generator being adapted to generate electrical waves whose instantaneous voltage values are proportional to the diflerences between the corresponding instantaneous fluid medium pressures on said diaphragm'surfaces, a second generator associated with the first named generator and adapted to generate electrical waves corresponding to the fluid medium waves approaching it from .any direction, the phase angle between the output voltage waves of the two generators being substantiallygreater than zero and substantially less than 180, and a circult to which the two generators areconnected comprising means adapted, when the generators are generating electrical waves corresponding to fluid medium waves approaching the apparatus from a given direction, to shift the phase angle between said output voltage waves to substantially 180 and combine the said waves in the latter phase relation.

18. In apparatus for generating electrical waves corresponding to fluid medium waves, the combination of a generator comprising diaphragm means having a pair of surfaces sensitive to the fluid medium' waves, said generator being adapted to generate electrical waves whose instantaneous voltage values are proportional to the differences between the corresponding instantaneous fluid medium pressures on said diaphragm surfaces, a second generator adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction and so disposed in relation to the first named generator that the fluid pressure upon it is zero when the difference in the pressures on the diaphragm surfaces of the first generator is a maximum, and a circuit to which the two generators are connected comprising means adapted to shift the phase relae tion between the electrical waves of the two generators substantially 90 degrees and to combine' the said waves.

19. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of a generator comprising diaphragm means having a pair of surfaces sensitive to the fluid medium waves and so disposed in relation to each other that the maximum time required for one of said waves to travel from a given point of one of said surfaces to a corresponding point of the other surface is less than the period of the shortest I wave within the said range of frequencies, said generator being adapted to generate electrical waves whose instantaneous voltage values are proportional to the differences between the corresponding instantaneous fluid medium pressures on said diaphragm surfaces, 9, second generator adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction and so disposed in relation to the first named generator that their electrical outputs are substantially 90degrees out of phase with each other over a given range of frequencies,

a and a circuit to which the two generators are connected comprising means adapted, within the said range of frequencies, to combine the outputs of the generators substantially 180 out of phase with each other when the generators are generating electrical waves corresponding to fluid medium waves approaching the apparatus from a given direction.

20. In apparatus for generating electrical v waves corresponding to fluid medium waves, the

combination of a bi-directional generator comprising means having a pair of surfaces sensitive to fluid medium waves, said generator being adapted to generate electrical waves in response to variations in the difference between the fluid medium pressures on said surfaces, a, second gonerator associated with vthe first named generator and adapted to generate electrical waves in response to fluid medium waves approaching it from any direction, circuit means for combining the outputs of the generators substantially 180 out of phase with each other when the generators are generating electrical waves in response to the fluid medium waves approaching the apteristic of the combined generators may be selected alternatively.

21. In apparatus for generating electrical waves corresponding to fluid medium waves, the combination of a bi-directional generator comprising means having a pair of surfaces sensitive to fluid medium waves, said generator being adapted to generate electrical waves in response to variations in the difference between the fluid medium pressures on said surfaces, a second generator adapted to generate electrical waves in response to the fluid medium waves approaching it from any direction and 30 disposed in relation to the first named generator that the fluid pressure upon it is zero when the difference in the pressures on the said surfaces of the first gener ator is a maximum, circuit means for combining the outputs of the generators substantially 180 out of phase with each other when the generators are generating electrical waves in response to the fluid medium waves approaching the apparatus from a given direction, and switching means for cutting out at will one of the two generators, whereby the directional characteristic of the other generator or the directional characteristic of the combined generators may be selected alternatively.

22. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of a generator comprising means having a pair of surfaces sensitive to the fluid medium waves and so disposed in relation to each other that the maximum time required for one of said waves to travel from a given point of one of said surfaces to a corresponding point of the other surface is less than the period of the shortest wave within the said range of frequencies, said generator being adapted to generate electrical waves in response to variations in the diflerence between the fluid medium pressures on said surfaces, a second generator associated with the first named generator and adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction, circuit means to which the two generators are connected adapted, within said range of frequencies, to combine the outputs of the generators substantially 180 out of phase with each other while the generators are generating electrical waves in response to fluid medium waves approaching the apparatus from a given direction, and switching means for cutting out at will one of the two generators, whereby the directional characteristic of the other generator or the directional characteristic of the combined generators may be selected alternatively.

23. In apparatus for generating electrical waves corresponding to fluid medium waves within a given range of frequencies, the combination of a generator comprising means having a pair of surfaces sensitive to the fluid medium waves and so disposed in relation to each other that the maximum time required for one of said waves to travel from a given point of one of said surfaces to a corresponding point of the other sur- -face is less than the period of the shortest wave within the said range of frequencies, said generator being adapted to generate electrical waves in response to variations in the difference be tween the fluid medium'pressures on said surfaces, a second generator adapted to generate electrical waves corresponding to the fluid medium waves approaching it from any direction and so disposed in relation to the first generator that the fluid pressure upon it is zero when the difference in the pressures on the diaphragm surfaces of the first generator is a maximum, circuit means to which the two generators are connected adapted, within said range of frequencies, to combine the outputs of the generators substantially 180 out of phase with each other when the generators are generating electrical waves in response to the fluid medium waves approaching the apparatus from a given direction, and switching means for cutting out at will one of the 'two generators, whereby the directional characteristic of the other generator or the directional characteristic of the combined generators may be selected alternatively.

ALFRED L. w. wnLmMs. 

